Is this right?

[Randall]

1963 HH Scott Stereomaster 380 home receiver/amp. http://hhscott.com/pdf/380.pdf

I want to check and maybe set bias on the 7591 outputs. There are test points on the cathodes for cathode bias measurement. I get roughly 240 mV on each side. I see the cathode T.P. goes across a 3.3 ohm 1 watt resistor to ground. Ohms Law states current equals voltage divided by resistance. So 240mV/3.3ohms = 72.7mA for two tubes, or 36.4 mA for one tube. The Weber Bias Calculator calls for 23.3mA at my 450 plate volts, and 15 watts dissipation at my cathode current of 36mA. Too high?

But I also see that the cathodes are tied to a 330K ohm resistor, and I'm not sure what that is for, or if it is skewing my methodology.

Now the thing is, I don't think the bias has been adjusted since the factory because this amp had all 21 original tubes. And it sounds great. So, do I just leave it, or should we strive for accuracy?

[Jazz P Bass]

Leave it!

[Dano]

Modifying circuits and amplifiers is something I refuse to do. In my tenure, doing so may create the scientific or physical reality, however sound is subjective to one’s own ears.

Changing the scientific and physical usually ended up in the client not liking the resultant sound.

It was written; “nothing sounds sweeter to a person’s ear than their own name.”

You wrote; “and it sounds great.”

I therefore second the motion.

Leave It!

Dan

[Randall]

I am not considering modifying anything, I want to know whether I should try to set the bias.

[JoeM]

It's been working fine all these years with the original tubes and it sounds great. Why do you feel the bias needs adjustment? I also vote for "Leave It"

[Randall]

Because I recapped it, including the bias supply, and thought it prudent to check the bias after. It seems like it is drawing too much current so I wanted to know if I am looking at this correctly.

[bob p]

The Weber bias calculator is likely to guide you into setting the bias higher than normal, because after all, it is a guitar amp biasing guide. What I would do if I were in your shoes is to look at the tube data sheet for your application, not the Weber bias tables. I'm too lazy to do that research for you, but it wouldn't surprise me if the amp is supposed to be biased colder than the Weber calculator would suggest.

The 330k resistors are grid bias resistors. They're elevated above circuit ground by being tied to the cathodes. If that isn't obvious to you, then it's probably best to just leave the amp alone.

[Randall]

bob p, I'm here to learn and ask for help, thank you. I looked at the data sheet before I came here, but I don't understand all the nomenclature. And if by my calculations my amp is drawing 36 mA and Weber calls for 23 mA, how is that colder? What am I missing?

[g1]

Well, I'll agree with everyone here but for different reasons . From what I can tell, most Scott amps with 7591 output tubes called for 70mA per pair of tubes (which is where you are at) so just leave it! H.H.Scott Amp Specs
But check that they are balanced as there is also an adjustment for that.
I'm sure most people here would rather see you go with the factory spec than some magic number that Weber says will apply to every amp (70%), thus the responses you received. And as BobP mentioned, that number is often hotter than required (though, as you replied, in this case the Weber number would be colder).

[Randall]

Thank you g-one. That's what I was looking for. I saw that amp spec chart but since it doesn't show current for my model, I didn't want to assume. I wasn't wanting to change anything I didn't have to, I just didn't know what value it is supposed to be, so I checked Weber and it didn't make sense. So I came here for guidance. And as always someone more knowledgeable than myself showed me the light. You will be happy to know the amp is now buttoned up after a full re-cap and a loud hum being traced to a PI tube, so I replaced the pair with NOS JAN/Phillips. Now I have to figure out where to find information on reading a tube chart better.